Question

giúp em câu 3 với em xin cảm ơn 

Answer 1

\(\dfrac{1+sin^2x}{1-sin^2x}-2tan^2x=\dfrac{1+sin^2x}{cos^2x}-2tan^2x=\dfrac{1}{cos^2x}+tan^2x-2tan^2x\)

\(=\left(1+tan^2x\right)-tan^2x=1\)